I like the idea of having a calendar for a given network. You could search based on location, cause, etc., to see when you can help out. “I have time Saturday morning. What is already scheduled that I could just show up to (with little or no preparation)?” This can also be a really cool way to promote BIG events. Get people hyped up for a big hooplah on a certain day.
Should also include a way to RSVP for events.